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REVIEW
Recent Advances of Graphitic Carbon Nitride-Based Structuresand Applications in Catalyst, Sensing, Imaging, and LEDs
Aiwu Wang1,2 . Chundong Wang1 . Li Fu3 . Winnie Wong-Ng4 . Yucheng Lan5
Received: 16 March 2017 / Accepted: 15 May 2017 / Published online: 8 June 2017
� The Author(s) 2017. This article is an open access publication
Highlights
• The g-C3N4-based materials have excellent electronic band structures, electron-rich properties, basic surface func-
tionalities, high physicochemical stabilities and are ‘‘earth-abundant.’’
• Recent progress of g-C3N4-based nanostructures in catalyst, sensing, imaging and LEDs have been discussed in details.
• An outlook on possible further developments in g-C3N4-based research for emerging properties and applications is also
included.
Abstract The graphitic carbon nitride (g-C3N4) which is a two-dimensional conjugated polymer has drawn broad
interdisciplinary attention as a low-cost, metal-free, and visible-light-responsive photocatalyst in the area of environmental
remediation. The g-C3N4-based materials have excellent electronic band structures, electron-rich properties, basic surface
functionalities, high physicochemical stabilities and are ‘‘earth-abundant.’’ This review summarizes the latest progress
related to the design and construction of g-C3N4-based materials and their applications including catalysis, sensing,
imaging, and white-light-emitting diodes. An outlook on possible further developments in g-C3N4-based research for
emerging properties and applications is also included.
Keywords Graphitic carbon nitride (g-C3N4) � Catalysis � Sensing � Imaging � LED
& Chundong Wang
& Yucheng Lan
1 School of Optical and Electronic Information, Huazhong
University of Science and Technology, Wuhan 430074,
People’s Republic of China
2 Center of Super-Diamond and Advanced Films (COSDAF)
and Department of Physics and Materials Science, City
University of Hong Kong, 83 Tat Chee Avenue, Kowloon,
Hong Kong SAR, People’s Republic of China
3 College of Materials and Environmental Engineering,
Hangzhou Dianzi Univerisity, Hangzhou 310018, People’s
Republic of China
4 Materials Science Measurement Division, National Institute
of Standards and Technology, Gaitherburg, MD 20899, USA
5 Department of Physics and Engineering, Morgan State
University, Baltimore, MD 21251, USA
123
Nano-Micro Lett. (2017) 9:47
DOI 10.1007/s40820-017-0148-2
1 Introduction
Graphitic carbon nitride (g-C3N4), one of the oldest
reported polymers in the literature, has a general formula of
(C3N3H)n. The history of development could be traced
back to 1834 [1]. Research work has been inspired in the
1990s due to a theoretical prediction that diamond-like b-
C3N4 could have extremely high hardness values [2]. At
ambient conditions, g-C3N4 is regarded as the most
stable allotrope. Similar to graphite, g-C3N4 is a layered
material in which van der Waals force holds the stacking
layers (covalent C–N bonds) and each layer is composed of
tri-s-triazine units connected with planar amino groups [3].
The tri-s-triazine ring structure provides the polymer a high
thermal stability (600 �C in air) and chemical stability in
both acidic and alkaline environments [4].
Utilization of g-C3N4 in the heterogeneous catalysis arena
started around a decade ago [5, 6]. The discovery of g-C3N4
polymer as a metal-free conjugated semiconductor photo-
catalysis for water splitting was first reported by Wang et al.
[7] due to its appealing electronic structure, i.e., having a
modulated bandgap and being an indirect semiconductor.
Since then, these unique properties of g-C3N4 make it a
promising candidate for visible-light photocatalytic appli-
cations utilizing solar energy. Solar energy is attracting
worldwide attention by providing about 120,000 TW annu-
ally to the earth as one of the green, clean, and sustainable
energy resources. Solar-induced chemical processes would
be able to greatly extend the applications of g-C3N4. Since
the landmark discovery of photocatalytic water splitting
using TiO2 electrodes by Fujishima in 1972, photocatalytic
technology has been regarded as one of the most important
strategies to address global energy and environmental issues
[8]. Since then, there have been numerous developments in
the fabrication of highly efficient semiconductor-based
photocatalysts such as metal-based oxides and sulfides
[9–12].
Notably, g-C3N4 has become a new family of next gen-
eration, non-toxic, metal-free, earth-abundant, and visible-
light-driven polymeric semiconductor for applications in the
degradation of organic pollutants, hydrogen evolution from
water, sensing, imaging, and energy conversion [5, 9,
13–144]. Many reviews can be found mainly focusing on
synthesis and catalytic applications of g-C3N4 [39–45,
145, 146]. However, a systematic description of the catalyst
(photo and organic), bio-imaging, (chemical and bio-)
sensing, devices, and energy-related applications (batteries,
supercapacitors, white-light-emitting diodes, and oxygen
reduction reaction) of g-C3N4 has not been presented until
now. In this review, we give an overview of the porosity,
luminescence, conductivity, and catalytic properties of
g-C3N4, as well as their bio-imaging, photodynamic therapy,
chemical sensing, and white-light-emitting diode applica-
tions. We believe this is the dawn for the development of
g-C3N4. There are still new physical properties yet to be
discovered based on g-C3N4 nanostructures. We are at a
critical time to highlight the progress and provide a good
source of references for this booming research topic.
2 g-C3N4-Based Structures
First-principle calculations predicted seven phases of g-C3N4,
namely a-C3N4 (bandgap of 5.5 eV), b-C3N4 (bandgap of
4.85 eV), cubic C3N4 (bandgap of 4.3 eV), pseudocubic C3N4
(bandgap of 4.13 eV), g-h-triazine (bandgap of 2.97 eV), g-o-
triazine (bandgap of 0.93 eV), and g-h-heptazine (bandgap of
2.88 eV) [4]. Figure 1 shows the primary tectonic units, tri-
azine and tri-s-triazine ring structures, for forming the allo-
tropes of g-C3N4. The structure can be viewed as graphite
N
N
N N
N N
N
N
(a) (b)
N
N NN
NN N
NNNN
N
N N
N
N
NH2
NH2
H2N N
N
N
N
N
N N
N
N
N
N
N N NNN
NN N
N
N N
N
NN
NN N
NN
N NN
NN N
NN
N NN
NN N
NN
N NH2
NH2
H2N N
NN N
NN
N
Fig. 1 a Triazine and b tri-s-triazine (heptazine) structures of g-C3N4. Reprinted with permission from Ref. [40]. Copyright 2016 American
Chemical Society
47 Page 2 of 21 Nano-Micro Lett. (2017) 9:47
123
whose carbon lattice is partially substituted with nitrogen
atoms in a regular fashion. g-C3N4 is an n-type semiconductor
[5–7], which intrinsically possesses a very high nitrogen
content dominated by a pyridinic and graphitic nitrogen while
supported by a two-dimensional (2D) graphene sheet or three-
dimensional (3D) porous graphitic carbon. The structure of
g-C3N4 can be controlled by a variety of synthetic routes,
including different condensation temperature, different ratio
of precursors, porosity induced by hard/soft templating
strategies, and exfoliation and doping. Synthetic routes and
various morphologies including bulk, mesoporous, 3D, 2D,
one-dimensional (1D), and zero-dimensional (0D) g-C3N4
will be discussed in the following.
2.1 Synthetic Routes of g-C3N4
g-C3N4 can be synthesized by thermal polymerization
of abundant nitrogen-rich and oxygen-free compound pre-
cursors containing pre-bonded C–N core structures (triazine
and heptazine derivatives) such as urea [46], melamine
[47–49], dicyandiamide [50–54], cyanamide [14, 35,
55, 56], thiourea [57, 58], guanidinium chloride [59–61,
125], guanidine thiocyanate [126], and thiourea oxide
[127]. The condensation pathways from above C/N
precursors are facile and efficient routes to form the poly-
meric g-C3N4 network [39]. Many reports discussed that
different types of precursors and treatments can strongly
influence the physicochemical properties of the resulting
g-C3N4, including surface area, porosity, absorption, pho-
toluminescence, C/N ratio, and nanostructures.
Various surface modifications and functionalities have
been employed to obtain desired structures such as 3D
bulks, 2D nanosheets, 2D films, 1D nanorods, 1D nan-
otubes, 1D nanowires, and 0D quantum dots. For example,
the urea precursor can be transformed to g-C3N4 at ca.
550 �C, as confirmed by X-ray diffraction (XRD, Fig. 2a,
b). The C3N4 powders are usually yellow under the visible
light. The optical properties are shown in Fig. 2c. The
polymeric g-C3N4 is unstable at above 600 �C. Beyond
700 �C, g-C3N4 produces nitrogen and cyano fragments.
General analytical techniques for confirming the pres-
ence of g-C3N4 include X-ray photoelectron spectroscopy
(XPS), XRD, and Fourier transform infrared (FTIR) spec-
troscopy [40]. The density function theory (DFT) calcula-
tions are used to reveal the characteristics of the valence
and conduction bands. For example, g-C3N4 is found to
mainly composed of the nitrogen pZ orbitals and carbon pZ
orbitals, respectively [5].
NN
NN N
N
NNNN
Tri-s-triazine (melem) unit
A perfect graphitic carbon nitride sheet Inte
nsity
(a.u
.)
2θ=13.0°, d1 =0.681 nm2θ=27.4°, d2 =0.326 nm
10 20 30 40 50 60 300
(a) (b) (c)
(d) (e) (f)(g) (h)
400
Abs
orba
nce
(a.u
.)
500Wavelength (nm)
600 700 8002 Theta (degree)
Fig. 2 Crystal structure and optical properties of graphitic carbon nitride. a Schematic diagram of a perfect graphitic carbon nitride sheet
constructed from melem units. b Experimental XRD pattern of the polymeric carbon nitride, revealing a graphitic structure with an interplanar
stacking distance of aromatic units of 0.326 nm. c UV-visible diffuse reflectance spectrum and image (inset) of g-C3N4. Reprinted with
permission from Ref. [5]. Copyright 2009 Nature Publishing Group. Representation of the b-C3N4 (d), a-C3N4 (e), graphite-C3N4 (f),pseudocubic-C3N4 (g), and cubic-C3N4 (h). The carbon and nitrogen atoms are depicted as gray and blue spheres, respectively, from Ref. [4].
Copyright 1996 American Association for the Advancement of Science. (Color figure online)
Nano-Micro Lett. (2017) 9:47 Page 3 of 21 47
123
2.2 Porosity of g-C3N4
High surface area and continuous porosity (as active
centers) are important requirements for catalysis, gas, and
energy storage technologies. Introduction of porosity in
g-C3N4 can significantly increase their exposed surface
area and accessible channels, and active edges, thus pro-
moting the charge separation, molecular mass transfer,
light harvesting, and surface reactions [145]. All these
advantageous features can benefit the enhancement of
photocatalytic efficiency. It is well known that the porous
carbons have outstanding properties with respect to their
use in energy applications, including as electrode mate-
rials for supercapacitors and as materials for solid-state
hydrogen and carbon dioxide storage. The attractive
attributions of porous carbons include low cost, environ-
mental friendliness, chemical and thermal stability, easi-
ness of processing, and low framework density. The
activated carbons have been traditionally employed as
absorbents or catalyst supports [62]. Compared with
activated carbons, g-C3N4 has a moderate nitrogen con-
tent and ideal stoichiometry. The nitrogen content induces
their unique surface properties such as semiconducting
character, mechanical stability, thermal and chemical
stability, which are superior to all other carbon nanoma-
terials (Table 1).
Cavities can be introduced to form mesoporous g-C3N4
frameworks [29]. Qiao’s group reported the preparation of
flexible films by integrating 2D mesoporous g-C3N4 (SiO2
template method) with graphene sheets. A commonly used
mesoporous silica, SBA15 was employed as a template to
fabricate mesoporous g-C3N4 [6, 56]. However, hydrogen
fluoride or other acid that are usually used to remove the
host matrices of SiO2 is hazardous. Besides hard template
methods [128–130], soft templates [131, 132] and bubble
templates [133–136] are also utilized to fabricate porous
g-C3N4. Liu et al. reported the preparation of porous
g-C3N4 by a simple co-pyrolyzation of co-precursors
melamine and NH4HCO3. NH4HCO3 not only enhanced
the specific area by bubbles, but it also shifted the con-
duction band and promoted the separation of charge car-
riers [63]. Calcium salts have also been utilized as a
template for the synthesis of porous g-C3N4 with enhanced
surface properties [143]. Metal–organic framework
(MOF) is a typical high surface area materials and has also
been utilized as templates for porous g-C3N4. Pandiaraj
et al. [64] have reported MOF-derived g-C3N4 porous
nanostructures.
2.3 Bulky g-C3N4
Bulky g-C3N4 can be synthesized by thermal condensation
of a variety of precursors such as cyanamide,
dicyandiamide, melamine, thiourea or urea between 400
and 600 �C. For example, g-C3N4 was synthesized from
cyanamide into a combination of addition and polycon-
densation, in which case the cyanamide molecules were
condensed to dicyandiamide and melamine at 203 and
234 �C, respectively. Next, the condensed dicyandiamide
was removed. Essentially, all melamine-based products
were found when the temperature was about 335 �C. Fur-
ther heating to about 390 �C resulted in the rearrangement
of tri-mesotriazine units via melamine. Finally, the polymer
g-C3N4 occurred at about 520 �C by further condensation of
the unit, which is thermally unstable at temperatures above
600 �C. During the calcination, the color changed from
white (precursor) to light yellow (500 �C) and then dark
orange (650 �C) [5, 40]. However, g-C3N4 obtained by such
methods usually possesses a relatively low surface area
(10 m2 g-1) and poor water solubility. Furthermore, the
bulk g-C3N4 does not exhibit any photoluminescence
characteristics when dissolved in solvents.
It has recently been found that urea is an excellent
precursor for the synthesis of flaky g-C3N4 having a high
specific surface area and a high porosity. g-C3N4 was
synthesized at various calcination temperatures between
450 and 600 �C in a muffle furnace for 2 h at a heating rate
of 15 �C min-1 from oxygen-containing urea. During the
thermal treatment process, the generated gases such as NH3
at a low temperature (\200 �C) and CO2 at a high tem-
perature play a leading role in processing highly porous
g-C3N4. The advantage of this method includes simplicity,
convenience, and the absence of introduction of impurities
during the synthesis of nanostructures. Compared to urea-
derived g-C3N4, comprising of wrinkled porous 2D
nanosheets, both thiourea-derived and dicyandiamide-
derived g-C3N4 samples showed large sheets without por-
ous structure. The specific surface area and crystallinity of
g-C3N4 were marginally improved with increasing calci-
nation temperatures. Generally, the heating rate is slower;
the porosity of g-C3N4 could be improved [40].
Exfoliation methods such as sonication have been
employed as a typical top-down route to obtain ultrathin
g-C3N4 with excellent photoluminescence properties [65].
Bulk materials can be swelled and then exfoliated in the
pure water. The dispersed ultrathin g-C3N4 nanosheets were
negatively charged (zetapotential of about *30.3 mV).
2.4 Three-Dimensional g-C3N4-Based Micro/
Nanostructures
Three-dimensional architectures fabricated using nano-scale
building blocks (0D, 1D, and 2D) are hot topics due to
the desirable combination of high internal reactive surface
area and straightforward molecular transport. However, the
47 Page 4 of 21 Nano-Micro Lett. (2017) 9:47
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Table 1 Recent reports on g-C3N4-based photocatalysts
Catalysts composition Precursors of g-C3N4 Photocatalyst applications Ref. (year)
g-C3N4 Cyanamide Hydrogen production [5] (2009)
g-C3N4/Graphene/NiFe Urea Photoelectrochemical [13] (2016)
g-C3N4 nanocapsules Cyanamide Hydrogen production [14] (2017)
g-C3N4/Co–N Urea Hydrogen production [16] (2016)
g-C3N4/Graphene Dicyandiamide Hydrogen production [18] (2014)
g-C3N4/PDA Melamine Hydrogen production [20] (2015)
Alkalinized-C3N4/Fe Melamine Photo degradation [21] (2016)
g-C3N4/Fe3O4 Melamine Photo degradation [22] (2013)
g-C3N4 Film Melamine Photoelectrochemical [24] (2015)
g-C3N4/AgBr Melamine Photo degradation [25] (2015)
g-C3N4 nanofibers Melamine Photo degradation [30] (2013)
g-C3N4/PNA Melamine Photo degradation [31] (2013)
g-C3N4 Film Cyanamide Photoelectrochemical [32] (2015)
P-doped g-C3N4 Melamine Hydrogen production [33] (2015)
Amorphous g-C3N4 Dicyandiamide Hydrogen production [34] (2015)
g-C3N4 Cyanamide Hydrogen Peroxide production [35] (2014)
g-C3N4/Ag/TiO2 Melamine Photo degradation [36] (2014)
g-C3N4/Bi Urea NO Purification [37] (2015)
g-C3N4/TiO2 Melamine Photoelectrochemical [47] (2016)
g-C3N4/ZIF Melamine CO2 Reduction [48] (2015)
N-doped g-C3N4 Melamine Hydrogen production [49] (2015)
Iodine modified g-C3N4 Dicyandiamide Hydrogen production [50] (2014)
Holey g-C3N4 Dicyandiamide Hydrogen production [51] (2015)
Phosphorylation g-C3N4 Dicyandiamide Hydrogen production [53] (2015)
Porous g-C3N4 Dicyandiamide Photo degradation [54] (2015)
g-C3N4 Cyanamide NO decomposition [55] (2010)
Mesoporous g-C3N4 Cyanamide Hydrogen peroxide production [56] (2015)
Porous g-C3N4 Thiourea Photo degradation [57] (2016)
S-doped g-C3N4 Thiourea and Melamine CO2 reduction [58] (2015)
g-C3N4/bismuth-based oxide Melamine or guanidine hydrochloride Photo degradation [61] (2016)
g-C3N4/Graphene Urea Hydrocarbon oxidation [66] (2016)
3D porous g-C3N4 Melamine Photo degradation [67] (2016)
g-C3N4 nanoplatelets Melamine Water splitting [73] (2015)
Graphene-like g-C3N4 nanosheets Dicyandiamide Hydrogen production [75] (2012)
Crystalline g-C3N4 Dicyandiamide Hydrogen production [76] (2014)
Sulfur-mediated g-C3N4 Trithiocyanuric acid Water oxidation [77] (2011)
Nanotube g-C3N4 Melamine Photo degradation [79] (2014)
Helical g-C3N4 Cyanamide Hydrogen production [80] (2014)
Nanorod g-C3N4 Cyanamide Hydrogen production and photoenzymatic catalysis [81] (2014)
Mesoporous g-C3N4 nanorods Cyanamide Hydrogen production and reduction of nitrophenol [82] (2012)
PAN/g-C3N4 Melamine Hydrogen production [83] (2016)
g-C3N4/ZIF Urea Photo degradation [84] (2017)
g-C3N4 Dicyandiamide Photo degradation [91] (2014)
g-C3N4/Pd Cyanamide Organic catalyst [92] (2015)
Oxidized g-C3N4 Melamine Organic synthesis [95] (2016)
g-C3N4/GO Melamine Photo degradation [98] (2014)
Nano-Micro Lett. (2017) 9:47 Page 5 of 21 47
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fabrication of 3D porous g-C3N4 has been a big challenge up
to now. Recently, Liu’s group developed an efficient
chemical vapor deposition growth strategy for 3D g-C3-
N4/graphene nanocomposites [66]. They found that g-C3N4
can be grown along the surface of graphene (see Fig. 3a, b,
c). The C–C bonds (284.7 eV) and N–C=N bonds
(288.3 eV) can be detected from the high-resolution C 1 s
spectrum (Fig. 3d). Furthermore, sp2 aromatic C=N–C
(398.8 eV), tertiary N-(C)3 (400.1 eV), and amino group C–
N–H (401.3 eV) can be fitted in the high-resolution N 1 s
spectrum (Fig. 3e). UV–vis spectroscopy (Fig. 3f) reveals
the optical properties of the 3D composites to be similar to
those of the pure g-C3N4.
Yuan et al. [67] reported the 3D porous g-C3N4 network
assembled by exfoliated ultrathin nanosheets intercon-
nected in large quantity via H2SO4 intercalation and sub-
sequent thermal treatment.
2.5 Two-Dimensional g-C3N4 Nanostructures
Two-dimensional materials have received tremendous
attention in the past decade because their ultimate structure
was reported by Geim [69]. The topic about graphene has
been cited over 30,000 times up to now (Google Scholar),
and graphene has been employed widely in energy appli-
cations (such as lithium-ion batteries and supercapacitors
[70–72]). Similar to graphene, g-C3N4 also has a typical
sp2 network (graphite-like layer structure) with weak van
der Waals interactions across the layers. Inspired by the
successful exfoliation of graphene from bulk graphite, Xie
et al. [65] firstly demonstrate that ultrathin g-C3N4
nanosheets could be prepared by a green liquid exfoliation
from bulk g-C3N4 in water. From bulk to ultrathin
nanosheets (several layers), g-C3N4 nanosheets show an
obvious increase in density of states (DOS) at the con-
duction band edge with respect to the bulk counterpart by
first-principle density-functional calculations. Global
effects have been launched to synthesize single or few-
layer g-C3N4 nanosheets due to their attractive physico-
chemical properties [52, 73, 74]. Figure 4 shows charac-
terization of the g-C3N4 nanosheets studied by transmission
electron microscopy (TEM) and atomic force microscopy
(AFM), illustrating the similarity of ultrathin layers of
exfoliated g-C3N4 to that of graphene.
There are two other synthetic approaches of 2D g-C3N4
nanosheets: one is known as the top-down approach, and
the other one is the bottom-up approach (Fig. 4e). For the
top-down approach, chemical etching and ultrasonication-
assisted liquid exfoliation are the two main technologies
involved [75, 76]. Template-assisted method and heteroa-
tom-mediated method have been commonly employed for
the bottom-up approaches [77, 78]. The big atomic size of
sulfur can influence the conformation and the connectivity
of the resultant g-C3N4 and hence offer a template tool to
tune the texture and electronic structure. The first report of
template-mediate synthesis of 2D g-C3N4 nanosheets
involved the use of GO-derived silica [78]. Zhang et al.
[23] reported the sol processing for the fabrication of
g-C3N4 thin films with HNO3 as a strong oxidizing acid.
Liu et al. [54] developed a method to grow g-C3N4 thin
films directly on conductive substrates.
2.6 One-Dimensional g-C3N4 Nanostructure
(Nanorods, Nanotube, and Nanofibers)
One-dimensional g-C3N4 nanostructures hold good pro-
mise for electronic and electrochemical performances due
to their high surface area, and light harvesting and mass
transfer properties. Wang et al. described a facile method
to fabricate g-C3N4 nanotubes (Fig. 5a) by directly
heating melamine without any templates. The resulting
nanotubes exhibited a blue fluorescence and excellent
photocatalytic properties [79]. Hollow 1D g-C3N4
nanostructures have been developed using a sulfur-me-
diated self-templating method by Liu’s group [83], as
shown in Fig. 5b.
Liu et al. [81] also reported the synthesis of g-C3N4
nanorods by using chiral silica nanorods as templates for
practical enzymatic applications. In addition, Li et al. [82]
described a one-step method to fabricate mesoporous
g-C3N4 nanorods with the template SBA-15. Tahir et al.
[30] reported the synthesis of g-C3N4 nanofibers for energy
storage and Photo degradation applications. Recently,
Tong et al. combined the hydrothermal and condensation
techniques to obtain a tubular g-C3N4 isotype heterojunc-
tion with excellent photocatalytic property [85]. Figure 5c–
f depicts these tubular nanostructures.
Zheng et al. [80] developed a nanocasting technique to
fabricate twisted hexagonal rod-like C3N4 by using chiral
silicon dioxide as templates. The helix is an important
template in nature, as one finds it in DNA, RNA, and
proteins. Synthesis of chiral inorganic nanostructures has
recently gained considerable attention. The chiral nanos-
tructures shown in Fig. 6a–f reveal the helical morphology
and ordered channels winding around the rod centers. This
is the first report about both left- and right-handed chiral
nanostructures with unique optical activities.
2.7 Zero-Dimensional g-C3N4 Nanostructure
When the size of the nanostructure is less than 10 nm,
g-C3N4 nanostructures (typically contain a few thousand
atoms) usually show significant quantum confinement
effects and possess excellent properties like bright fluo-
rescence, water solubility, and above all, non-toxicity. This
is in contrast to the fact that most essential elements in
47 Page 6 of 21 Nano-Micro Lett. (2017) 9:47
123
semiconductor quantum dots (QDs) (for example, Se in
CdSe, Pb in PbTe, and Te in CdTe) all have risks of long-
term toxicity and potential environmental hazards. Similar
to the synthetic routes of 2D g-C3N4 nanosheets, g-C3N4
QDs have been mainly synthesized by top-down and bot-
tom-up approaches.
(d)
(a) (b) (e)
(c)
200 nm200 nm 100 nm100 nm
500 nm500 nm
4
3
2
1
0
5 nm5 nm
nmnm
bulk CN solids
Top-down lay-by-lay exfoliation
CNNs
precursors
Bottom-up anisotropic assembly
Fig. 4 a-c TEM images and d AFM image of g-C3N4 nanosheets. Inset of c SAED pattern. Reprinted with permission from Ref. [52]. Copyright
2014 John Wiley and Sons. e Schematic illustration of top-down and bottom-up synthetic strategies for g-C3N4 nanosheets. Reprinted with
permission from Ref. [68]. Copyright 2015 The Royal Society of Chemistry. (Color figure online)
C3N4
Graphene
Graphene/C3N4
Graphene
5 μm 2 μm 10 nm
2L
(c)(b)(a)
(d) (e) (f)
G-supported C3N4
200 nm
C –C
284.7
398.8
C –N
286.9 401.3
N –C =N
288.3
404.3
Raw data
282 284 286 288 290Binding energy (eV)
Inte
nsity
(a.u
.)
Inte
nsity
(a.u
.)
Abs
orba
nce
(a.u
.)
292 396 398 400 402 404 406Binding energy (eV)
Fitting
C =NC=N –C
C–N –HRaw dataFitting
N − (C)3
GCN-0.8GCN-4
C3N4
GCN-16
285.7
400.1
300 400 500 600 700 800
G content increase
Wavelength (nm)
Fig. 3 a Schematic 3D g-C3N4/Graphene structures. b, c SEM images of g-C3N4/graphene nanocomposites. Inset TEM image. d C 1s, e N
1s XPS and f UV–vis spectra of g-C3N4/graphene nanocomposites. Insets: photographs of the powder samples. Reprinted with permission from
Ref. [66]. Copyright 2016 American Chemical Society
Nano-Micro Lett. (2017) 9:47 Page 7 of 21 47
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g-C3N4 QDs were prepared first by the top-down
approach. Wang et al. was the first to prepare g-C3N4 QDs
by a thermal-chemical etching process from bulk g-C3N4,
as illustrated in Fig. 7e [86]. Xie et al. demonstrated an
exfoliation strategy for the preparation of single-layered
QDs. When these QDs passed through the cell membranes,
they exhibited an excellent two-photon absorption behavior
as compared with the double-layered QDs [87] (Fig. 7a–d).
Recently, Wu et al. developed a doping method (phos-
phorus as dopant) to control the emission wavelength of
g-C3N4 QDs to be in the whole visible-light regime [88].
This was the first report that phosphorus doping could
change the direct bandgap of g-C3N4 QDs.
Hydrothermal and microwave heating methods are
commonly utilized for the synthesis of g-C3N4 QDs as the
bottom-up approach [89, 90]. Lu et al. [90] reported the
synthesis of g-C3N4 QDs with strong blue photolumines-
cence by hydrothermal heating of the citric acid and
thiourea. Cao et al. [89] developed a facile microwave-
assisted fabrication of g-C3N4 QDs. Compared with the
majority of carbon materials such as graphene QDs and
carbon QDs, g-C3N4 QDs which possess both nitrogen-rich
and electron-rich properties and basic surface functionali-
ties represent a new family of luminescent QDs.
3 Multifunctional Applications
As the most stable allotrope of carbon nitride, g-C3N4 is
versatile materials with unique semiconducting, excellent
photocatalytic properties. They are also environmental
friendly, low cost, and metal-free, which make them
attractive for a range of applications beyond catalysis,
including sensing, bio-imaging, photodynamic therapy, and
energy conversion.
3.1 g-C3N4 Catalysts
Polymeric g-C3N4 semiconductors are widely used as cat-
alysts due to their excellent chemical stability and unique
400 nm400 nm 200 nm200 nm 100 nm100 nm
10 μm10 μm50 nm50 nm 1 μm1 μm
0 2 4Pore radius (nm)dV
/dD
(cm
3 g−1
nm
−1)
6 8 10 12
(d)
(e) (f)
(c)(b)(a)
Fig. 5 1D g-C3N4 nanostructures. a SEM image of nanotubes [79]. b TEM image of nanotubes [83]. c TEM image of nanorods [81]. d TEM
image of porous nanorods [82]. Inset: pore size distribution. e SEM image of nanofibers [30]. f SEM image of tubular structures [85]. Reprinted
with permission from Ref. [79] (Copyright 2014 The Royal Society of Chemistry), with permission from Ref. [83] (Copyright 2016 American
Chemical Society), with permission from Ref. [81] (Copyright 2014 American Chemical Society), with permission from Ref. [82] (Copyright
2012 The Royal Society of Chemistry), with permission from Ref. [30] (Copyright 2013 American Chemical Society), with permission from Ref.
[85] (Copyright 2016 Wiley–VCH Verlag GmbH), respectively
47 Page 8 of 21 Nano-Micro Lett. (2017) 9:47
123
electronic band structure. In the formation of the g-C3N4
network, the C-pz orbit composes the lowest unoccupied
molecular orbital (LUMO), and N-pz orbital composes the
highest occupied molecular orbital (HOMO), with a 2.7 eV
bandgap between these two orbitals [91]. This suit-
able bandgap can absorb the solar electromagnetic energy
with wavelength less than 475 nm. It was found that pyr-
rolic nitrogen has the strongest role in acetylene
hydrochlorination among all nitrogen species [17].
The rapid recombination rate of electron–hole pairs
results in low efficiency, thus limiting the practical appli-
cations of g-C3N4. Sun et al. [92] developed a homogeneous
catalyst, Pd/g-C3N4, for a Suzuki–Miyaura coupling reaction
with superior catalytic activity under mild conditions. It is
well known that the Suzuki–Miyaura coupling reaction is of
primary importance for the construction of C–C bonds. The
uniform Pt nanoparticles deposited on the surface of g-C3N4
networks can result in a high yield of 97% biphenyl and
100% bromobenzene. Kumar et al. reported a nanocom-
posite of an iron(II) bipyridine with carbon nitride as a
photocatalyst for the oxidative coupling of benzylamines
under mild reaction conditions, resulting in excellent activity
and effective recycling ability [93]. A photoactive catalyst
Ru/g-C3N4 was developed by Sharma et al. [94] for efficient
photocatalytic transfer hydrogenation of aldehydes and
ketones under mild conditions. Xie’s group explored the
generation of singlet oxygen in oxidized g-C3N4 [95].
g-C3N4 can be used as a new kind of metal-free pho-
tocatalysts. Wang et al. [5] was among the first to use
g-C3N4 as a photocatalyst for hydrogen production from
water. However, the quantum efficiency of the catalyst is
only 0.1% with the irradiation of 420–460 nm due to its
fast recombination of electron–hole pairs. To solve this
problem, modified 2D g-C3N4 materials with a redshift
absorption were produced and a quantum efficiency of
8.8% was achieved at 420 nm by the same group [96]. Liu
et al. [97] developed a carbon dots/g-C3N4 nanocomposite
as a metal-free photocatalyst with high quantum yield and
excellent stability. The overall evolution of H2 and O2 is
shown in Fig. 8a with a molar ratio of 2.02 (a value of 2 is
identified for water splitting). Absorbance and quantum
efficiency (QE) of the carbon dots/g-C3N4 nanocomposite
were measured and are shown in Fig. 8b. The catalyst
composition was optimized by measuring QE for different
concentrations of carbon dots in a fixed mass of composite,
as shown in Fig. 8c. With the increase in carbon dots, the
QE can reach as high as 16% (Fig. 8d).
(a) (b) (c)
(d)
(e) (f)
C
N
500 nm 200 nm
Fig. 6 Morphology characterization of the HR-CN sample. a SEM,
b TEM, and c, d corresponding elemental mapping images of g-C3N4.
Reproduced from Ref. [80] by permission of the John Wiley & Sons
Ltd. e A g-C3N4 layer and f A single g-C3N4 nanotube formed by
rolling the g-C3N4 layer. Reproduced from Ref. [79] by permission of
the Royal Society of Chemistry
(a) (b) (e)
(c) (d)
single-layer
double-layer
Bulk g-C3N4 g-C3N4 nanosheets
g-C3N4 nanoribbonsLUMO+nHOMO-n
carbonnitrogen
g-C3N4 quantum dots
Hydrothermalcutting
Heat etching Acidic cutting
Fig. 7 a HOMO-n and b LUMO ? n orbitals of the single-layered g-C3N4, respectively. c HOMO-n and d LUMO ? n orbitals of the double-
layered g-C3N4, respectively. Reproduced from Ref. [87] by permission of John Wiley & Sons Ltd. e Schematic illustration of the controllable
synthesis of g-C3N4 nanosheets, nanoribbons, and quantum dots. Reproduced from Ref. [86] by permission of the Royal Society of Chemistry
Nano-Micro Lett. (2017) 9:47 Page 9 of 21 47
123
Recently, nanohybrids (van der Waals heterostructures)
which compose of different 2D nanolayers exhibit much
improved catalytic activities. For example, porous g-C3-
N4/graphene films have been fabricated as electrodes for
efficient hydrogen evolution by Qiao et al. [29]. Dai et al.
[98] obtained graphene oxide/g-C3N4 nanosheets by a
sonication method with reinforced photocurrent. Ma et al.
demonstrated the fabrication of hybrid g-C3N4/graphene
quantum dot nanocomposites. The hybrid nanocomposites
have an excellent efficiency for water splitting due to
decreased bandgap. Experimental results and DFT calcu-
lations revealed that the chemical bonding of two different
layered materials can improve the catalytic activity [18].
Xu et al. [137] fabricated AgBr/g-C3N4 hybrid materials
with synergistic visible-light photocatalytic activity, and
the uniform AgBr nanoparticles were well dispersed on the
g-C3N4 nanosheets which enhanced the optical activity.
What is more, many hybrid materials like g-C3N4/Ag2CO3
[138], g-C3N4/ZnWO4 [139], g-C3N4/Ag2O [140], and
g-C3N4/Ag [141] were explored to enhance photocatalytic
activity. Sun group obtained mesoporous carbon/g-C3N4
with remarkably enhanced photocatalytic activity due to
enhanced visible light and dye adsorption [142]. Meso-
porous g-C3N4 has been fabricated in the presence of SBA-
15 with enhanced photocatalytic activity for methyl orange
degradation by Gao et al. [144].
Crystalline carbon nitride nanosheets have been pre-
pared by Lotsch et al. [76] to enhance visible-light
hydrogen evolution. The authors stated that morphology
and surface area are the two crucial factors governing the
photocatalytic performances. An efficient deposition
method of growing g-C3N4 on different electrodes has been
developed by Shalom et al. [26]. The successful deposition
technique enables the fabrication of many electronic
devices based on g-C3N4. Wu et al. studied the effect of
defects in g-C3N4 on hydrogen evolution and photovoltage.
Controlling different types of defects is the key to improve
the catalytic performance [27].
He et al. [20] utilized polydopamine/g-C3N4 composites
to produce hydrogen from water with superior activity.
DFT calculations were used to obtain the band structure of
g-C3N4 with nonmetal element doping, including boron,
oxygen, phosphorous, and others. Phosphorus was pre-
dicted to be suitable as a doping element in g-C3N4 because
it can decrease the bandgap of g-C3N4 from 2.7 to 2.31 eV
without any mid-gap states [28].
Shiraishi et al. [35] reported a g-C3N4 photocatalyst to
reduce O2 to H2O2 via a two-electron route under visible
light. Fang et al. [49] demonstrated that the nitrogen self-
doped g-C3N4 can significantly enhance the catalytic
activity (1.8 times) of hydrogen evolution and modify the
optical and electronic properties with respect to the un-
8.4μmol/h
Gas
es e
volu
tion
(μm
ol) 200
150
100
50
01st 5th
1086420
16
12
8
4
0
30thCycle number Wavelength (nm)
Abs
orba
nce
(a.u
.)
200th
0
0 2.0×10−3 4.0×10−3 6.0×10−3 8.0×10−3 0 0.2 0.4CDots-C3N4 catalyst/water (g/L)
0.6
4 8CDots concentration (×10−5 g/g)
CDots concentration (g/g)
12 16
400
15
12
9
6
3
0
500 600 700 800
0(a) (b)
(c) (d)
1 day 5 days 30 days 200 days
4.1μmol/h
H2
O2 O2 O2
8
6
4
2
0
O2
H2 H2 H2
Qua
ntum
eff
icie
ncy
(%)
QE
(%)
QE
(%)
QE
(%)
Fig. 8 a Typical H2 and O2 production from water under visible-light irradiation. b Wavelength-dependent QE (red dots) of water splitting by
composite catalyst. c QE for different concentrations of carbon dots/g-C3N4 catalysts in a fixed mass of composite catalyst. d QE for different
catalyst loads with a constant carbon dot concentration in 150 ml of ultra-pure water. Reproduced with permission from Ref. [95]. Copyright
2015 American Association for the Advancement of Science. (Color figure online)
47 Page 10 of 21 Nano-Micro Lett. (2017) 9:47
123
doped g-C3N4. Sun’s group reported ultrathin g-C3N4
nanosheets/graphene nanocomposites as a highly efficient
electrocatalyst for oxygen evolution reaction. They revealed
that the high oxygen evolution reaction activities resulted
from pyridinic-N-related active sites [121]. They also
developed 3D porous supramolecular architecture based on
g-C3N4 nanosheets/graphene oxide as a highly efficient
electrocatalyst for oxygen reduction reaction [122].
Besides metal-free materials such as graphene or carbon
dots, various metal oxides and sulfides have been coupled
with g-C3N4 for enhancing photocatalytic performances.
For example, Chen et al. demonstrated that the g-C3N4/Ag/
TiO2 heterostructure microspheres were successfully
achieved with enhanced photocatalysis performances [36].
Gu et al. [100] obtained the g-C3N4/TiO2 nanosheets with
high reactive (001) facets by a hydrothermal method,
accompanied by a remarkable enhancement of photocat-
alytic capability in degradation of organic molecules under
visible and UV light. Wang et al. successfully fabricated
g-C3N4/BiPO4 g-C3N4 photocatalyst. The hybrid structure
has been proved by Dong et al. to be a novel photocatalyst
for the application of NO purification via an in situ depo-
sition method [37]. Wang et al. explored the enhanced
photocatalytic mechanism for the hybrid g-C3N4/MoS2
nanocomposites by DFT calculations. DFT calculations
show these hybrid catalytic nanocomposites indeed have a
higher absorption (used in the treatment of methylene
orange (MO) [101]), with a decent photocatalytic perfor-
mance and better separation of photo-generated carriers
[38]. Ye’s group has demonstrated a zirconium-based
metal–organic framework (Uio-66)-g-C3N4 nanosheet
compound via a facile self-assembly method. Photocat-
alytic CO2 reduction activities were greatly enhanced. The
photo-generated electron can be transferred from the
g-C3N4 nanosheets to Uio-66 for better reduction of CO2
[48].
Ye’s group was inspired by the natural photosynthesis
process and explored an environmental ‘‘phosphorylation’’
strategy to improve the photocatalytic performance [48]. A
hydrogen generation rate of 947 lmol h-1 and a quantum
yield of 26.1% at 425 nm were achieved. This is the highest
record for g-C3N4-based photocatalyst [53]. Recently, Tong
et al. were inspired by the function of thylakoids in a natural
photosynthesis system as shown in Scheme 1a. They suc-
cessfully fabricated a multishell g-C3N4 nanocapsule pho-
tocatalysts with enhanced light harvesting and electron
transfer properties. The overall synthesis procedure is
illustrated in Scheme 1b. The triple-shell g-C3N4 can pro-
duce hydrogen as much as 630 lmol h-1. This success
potentially produces a new generation of solar devices for
hydrogen production.
Molecular doping not only plays an indispensable role in
regulating the bandgap and electron structure of g-C3N4,
but also controls the physical and chemical properties of
g-C3N4. The bottom-up copolymerization method allows a
large selection of structurally matched organic anchoring
groups to be integrated into the g-C3N4 tri-triazine back-
bone to design highly efficient photocatalysts with the
desired chemical composition and bandgap. It is expected
that the modified g-C3N4 nanocrystals will provide an
insightful view of the sustainable use of solar energy in
chemistry because of the interesting new features that
thylakoid
CO2
H2OO2
Natural photosynthesis
SiO2 SiO2/CY
III Na2CO3
I cyanamide
(b)
(a)
II 550 °C, N2, 3 h
MSCN
Scheme 1 a Natural photosystem with green leafs, and the enlarged figure (right) depicts the light conversion in the stacked thylakoids.
b Schematic illustration for the preparation of MSCN nanocapsules. Adapted with permission from Ref. [14]. Copyright 2017 American
Chemical Society
Nano-Micro Lett. (2017) 9:47 Page 11 of 21 47
123
provide a new avenue for the study of heterogeneous
catalysis.
g-C3N4 has a moderate bandgap of 2.7 eV, which makes
it active in the visible region. Thermodynamic losses and
overpotentials are usually considered in the photocatalytic
process, the bandgap of 2.7 eV lying in between 2 eV (water
splitting with enough endothermic driving forces) and
3.1 eV (visible-light absorption). What is more, g-C3N4 has
a suitable conduction band position for various reduction
reactions. The CB of g-C3N4 is more negative than H2-
evolution, CO2-reduction, and O2-reduction reactions,
revealing that the photo-generated electrons in g-C3N4
possess a large thermodynamic driving force to reduce small
molecules like H2O, CO2, and O2. Therefore, the suit-
able electronic band structures of g-C3N4 are favorable for
its applications as catalyst, such as water splitting, CO2
reduction, oxygen reduction reaction, oxygen evolution
reaction, pollutant degradation, and organic synthesis.
Advantages and disadvantages Bandgap is crucial for
photocatalytic applications. g-C3N4 has a direct moderate
bandgap of 2.7 eV which has visible-light activity besides
its advantages of low cost, metal-free, 2D layered structure,
easy fabrication, and high stability. Unfortunately, the bulk
g-C3N4 generally exhibits the low photocatalytic efficiency
due to some serious drawbacks of g-C3N4 material, such as
the high electron–hole recombination rate, low surface
area, insufficient visible absorption, few active sites for
interfacial reactions and low charge. Among various design
strategies, heterojunction construction (especially for
Z-scheme construction) and porosity design (microporous,
mesoporous, and macroporous) have been employed.
Based on present g-C3N4 materials, photocatalytic CO2
reduction seems to be more promising in developing
practical g-C3N4-based photocatalysts.
3.2 g-C3N4 Sensing
It is well known that the polymeric g-C3N4 exhibits pho-
toluminescence (PL) properties similar to many semicon-
ductors materials. g-C3N4 emits a blue PL around 450 nm
when dissolved in solvents under UV light irradiation due
to its direct bandgap of 2.7 eV, which can be explained as
the transition of the s-triazine ring [39]. Luminescent
g-C3N4 (nanosheets and nanodots) can simply be regarded
as nitrogen-rich carbon dots, although the quantum yield of
g-C3N4 (up to 40%) is lower than carbon dots (up to 90%)
[102]. g-C3N4 nanostructures exhibit a higher stability than
the carbon dots, therefore potentially providing more
practical applications. However, we are still far from the
success of improving the quantum yield and understanding
the precise PL mechanism of g-C3N4.
Based on the unique PL property of g-C3N4, g-C3N4
nanosheets have a strong response to copper ions [74] as
turn-off chemical sensors. Since the chemical reduction of
Cu2? to Cu? lies between the conduction band and valence
band of g-C3N4, the PL of g-C3N4 can be quenched with a
low detection limit of 0.5 nM [103]. A similar mechanism
has been explained in previous work [104]. In that work, a
relatively low detection limit of 0.04 nM has been achieved
by CdTe QDs. Besides copper ions, PL of g-C3N4 can also
be quenched by other metal ions like Fe3?, Ag?, Hg2?, and
Cr2? [89, 90, 105–110]. Huang et al. reported the fabrica-
tion of g-C3N4 nanosheets for the selective detection to
Cu2? and Ag?. Zhang et al. successfully prepared the
g-C3N4 QDs as effective fluorescent probes for the detec-
tion of Fe3? and Cu2?. Cao et al. developed the g-C3N4
nanodots via a microwave-assisted approach. The produced
nanodots were utilized as turn-off sensors for mercury ions
with a detection limit of 0.14 lM [89]. Shao’s group suc-
cessfully synthesized the oxygen- and sulfur-co-doped
g-C3N4 nanodots via a hydrothermal method, which has a
lower detection limit of mercury ions (0.37 nM) [90]. Sun’s
group successfully fabricated Fe-doped g-C3N4 nanosheets
with a highly sensitive optical detection of glucose due to
the chelation of Fe3? with N [123]. Moreover, they
demonstrated the ultrathin g-C3N4 nanosheets can be uti-
lized as electrochemical glucose biosensing with a detection
limit of 11 lM [124]. Rong et al. explored turn-off–turn-on
sensors using g-C3N4 nanosheets, Cr and ascorbic acid.
After Cr quenches the PL signal of g-C3N4, the addition of
ascorbic acid can recover the PL signal due to the oxida-
tion–reduction between Cr and ascorbic acid. Yang’s group
fabricated a g-C3N4 nanosheet/MnO2 sandwich nanocom-
posite via a one-step approach. The fabricated composites
could turn-on the fluorescent probes of glutathione (GSH)
with a high selectivity due to fluorescence resonance energy
transfer (FRET), as shown in Scheme 2a [111]. GSH is
possibly a suitable recovering agent of the PL signal of
g-C3N4 as well. Xu et al. reported a g-C3N4/Hg system
without PL signal, which can be switched on by an intro-
duction of GSH. The system may be employed for detection
of GSH in various food samples [112]. Based on FRET,
g-C3N4 was found to detect riboflavin (RF) because g-C3N4
acts as a donor of energy transfers and RF as an acceptor
[113]. A turn-on g-C3N4-based long-persistent luminescent
probe for detection of biothiols was reported by Tang et al.
[115] for the first time. This long-persistent luminescence
allows the detection without external illumination. Qiao’s
group has successfully prepared the proton-functionalized
ultrathin g-C3N4 nanosheets which can interact with hep-
arin, therefore achieving the lowest detection limit of
18 ng mL-1 [52].
Feng et al. successfully fabricated Au-nanoparticle-
functionalized g-C3N4 nanosheets coupled with Ru(bpy)32?.
The coupled g-C3N4 nanosheets can be employed for RNA
detection based on dual-wavelength
47 Page 12 of 21 Nano-Micro Lett. (2017) 9:47
123
electrochemiluminescence (ECL), as shown in Scheme 2b.
Au/g-C3N4 composites exhibit an emission at 460 nm and
g-C3N4/Ru(bpy)32? at 620 nm. Here, g-C3N4 acts as a donor
of energy transfer and Ru(bpy)32? as an acceptor for highly
sensitive and selective detection of target miRNA (ECL
signals quenching at 460 nm and increasing at 620 nm)
[114].
In addition to detecting metal ions and bio-molecules by
g-C3N4, g-C3N4 can also respond to temperature [116].
Debanjan et al. reported a temperature sensor based on the
PL of g-C3N4. They found that as the temperature
increased, the intensity of PL decreased.
3.3 g-C3N4 Imaging
Non-toxicity, metal-free, high stability, and high PL
quantum yield enable g-C3N4 nanosheets and nanodots to
be promising candidates for cell imaging. Xie’s group
KMnO4(a)
(b)
KMnO4
MES bufferultrasound
Glutathione
No Glutathione
g-C3N4 nanosheet
Au-g-C3N4 NH
g-C3N4-MnO2 nanocompositeET ET
hv
hv PL
460 nm
ECL at 460 nm
ECL-RET
(Ru(bpy)32+)*
Ru(bpy)32+
Off ECL emission
On ECL emissionProbe DNADSNTarget microRNAMCHMolecular beacon
Ru(bpy)32+
(g-C3N4)*
g-C3N4
SO4
ECL at 620 nm
620 nm
Cycle
ECL
−.
−.
Scheme 2 a Schematic representation of g-C3N4/MnO2 nanocomposite for sensing of GSH. Reproduced with permission from Ref. [107].
Copyright 2014 American Chemical Society. b Schematic illustration of the dual-wavelength ratiometric ECL-RET biosensor configuration
strategy. Reproduced with permission from Ref. [114]. Copyright 2016 American Chemical Society
Nano-Micro Lett. (2017) 9:47 Page 13 of 21 47
123
demonstrated the preparation of ultrathin g-C3N4 nanosh-
eets for bio-imaging applications [65]. They found that
g-C3N4 nanosheets have no significant effects on the HeLa
cell viability even at a high concentration. The same group
further developed the single-layered g-C3N4 QDs for both
one-photon and two-photon cell imaging as long as the
QDs can pass through the nuclear pore and enter into the
nuclei [87]. Singlet oxygen, being one of the most impor-
tant reactive oxygen species (ROS), could be generated in
the presence of g-C3N4 as a photosensitizer [95].
Recently, Yang’s group reported the NIR-driven
g-C3N4/up-conversion nanoparticle (UCNP) composite for
efficient bio-imaging and photodynamic therapy (PDT)
[117]. In this application, g-C3N4 functions as a sensitizer
to absorb the UV light converted by UCNPs from 980 nm
NIR light. The generated ROS causes the tumor to shrink
or disappear effectively without any side effects from the
irradiation [117]. However, long-time irradiation of
980 nm NIR light can cause overheating of tissues; there-
fore, an 808 nm laser light is more suitable for the PDT.
Feng et al. [118] fabricated a novel core–shell structure
(UCNP/g-C3N4) for phototherapy and imaging applica-
tions. Mesoporous g-C3N4 was coated outside of the shell
of UCNPs, generating a large amount of ROS due to the
large surface area of g-C3N4 under the 808 nm laser
irradiation. In vivo experiments were conducted as
shown in Fig. 9. The factors affecting the efficiency of
sensing and imaging are mainly the quantum yield,
functionalization, PL and optical properties, stability, and
toxicity.
**
UCNPs@g-C3N4-PEG + 808 nm NIRUCNPs-PEG + 808 nm NIRUCNPs@g-C3N4-PEGNIRControl
UCNPs@g-C3N4-PEG + 808 nm NIRUCNPs-PEG + 808 nm NIRUCNPs@g-C3N4-PEGNIRControl
12
10
8
6
4
2
0
32
30
28
26
24
22
Rel
ativ
e tu
mor
vol
ume
(V/V
0)
0 2 4 6 8 10 12Time (day)
14 16 18
0
Nor
mal
ized
bod
y w
eigh
t (g)
2 4 6 8 10 12Time (day)
(e)
14
ControlControl 808 nm laser808 nm laserUCNPs-PEGUCNPs-PEG808 nm laser808 nm laserUCNPs@g-CUCNPs@g-C3N4-PEG-PEG
UCNPs@g-CUCNPs@g-C3N4-PEG-PEG808 nm laser808 nm laser
Control 808 nm laser UCNPs-PEG808 nm laser
UCNPs@g-C3N4-PEG UCNPs@g-C3N4-PEG808 nm laser
100 μm 100 μm 100 μm
(a)
(b)
(d)
(c)
100 μm 100 μm
Fig. 9 a In vivo tumor volume growth curves of mice in different groups after various treatments. b Body weight changes of Balb/c mice versus
treated time under different conditions. c Photographs of excised tumors from representative Balb/c mice after 14 day treatment and d the
corresponding digital photographs of mice in the control group and ‘‘UCNPs@g-C3N4 - PEG with 808 nm laser’’ group after 14 day treatment.
e H&E stained tumor sections after 14 day treatment from different groups. Reproduced with permission from Ref. [118] Copyright 2016
American Chemical Society
47 Page 14 of 21 Nano-Micro Lett. (2017) 9:47
123
3.4 g-C3N4-Based LED
Although the PL properties of g-C3N4 have been investi-
gated in the past fifteen years, the solid-state lighting of
g-C3N4-based materials is still at an infancy stage. Various
investigations about the g-C3N4-based solid-state lighting
such as white-light-emitting diodes (WLEDs) have been
carried out. Wang et al. fabricated the g-C3N4/silica gels
for WLEDs application (Fig. 10a–d). In this work, using a
one-step heat treatment approach confirmed by the FTIR,
g-C3N4 was found to be covalently bonded with silica gels
(Fig. 10e, f). The g-C3N4/silica gels obtained possess
emerging properties with respect to bare g-C3N4, including
water-resistance, high transparency, high flexibility, and
white light emission under UV irradiation. The mechanism
for white light emission can be ascribed to surface passi-
vation by silica [119]. Bayan et al. [120] prepared a g-C3N4
sheet/ZnO nanorod hybrid for WLEDs by combining the
emissions of g-C3N4 and ZnO nanorods to achieve a broad
emission. Last but not the least, Gan et al. studied the
origins of broad PL of g-C3N4 for the WLEDs applications.
They demonstrated that the broad PL from g-C3N4 is
attributed to band-to-band transitions in the tri-s-triazine
rings. This novel work can help the understanding of the
PL mechanism and accelerate the progress of WLEDs-
based g-C3N4.
4 Conclusion and Outlook
This review summarizes the recent advances of g-C3N4-
based structures and applications including catalyst,
chemical and biosensing, imaging, and LEDs. The per-
formances of g-C3N4 are mainly based on their surface
state (defects, function groups, and doping) and structures
(porosity, thickness, and morphology). Although a signifi-
cant advancement has been made for the development of
highly efficient g-C3N4-based photocatalysts, there are still
considerable problems that require further investigations,
including the catalytic rate and design.
2D polymeric g-C3N4 materials featuring low cost,
metal-free, environmental friendly, moderate bandgap,
high chemistry activity, and high stability have only been
studied for the past few years (from fundamental research
400 500 600 700
(a) (b)
(c)
(d) (f)
(e)
450 550 650
400 500 600 700Wavelength (nm)
PL In
tens
ity (a
.u.)
450 550 650
g-C3N4-silica gels 365 EX
Pure C3N4 365 EXPure C3N4 430 EXPure C3N4 480 EXPure C3N4 580 EXAEATMS 365 EX
0.90.80.70.60.50.40.30.20.1
00 0.2 0.4
Tran
smitt
ance
(a.u
.)
0.6 0.8x
y600
580
560
540
520
500
480
460
620
4000 3000
g-C3N4
g-C3N4 silica gels
AEATMS
3500 2500 15002000 1000 500
O
O
HNHN
OO
O
OO
O
NHNH
O
O
O
O
NH HN
HN
HN
HNN
H
NHNH
Si Si
Si
Si
Si
Si
Wavenumber (cm−1)
1638
1655
1709
1123
11861035
Fig. 10 a Photoluminescence spectrum of the g-C3N4/silica gels excited at 365 nm displaying four peaks (430, 480, 580, and 627 nm) in the
visible regime. b, c Photographs of a free-standing g-C3N4/silica-gel membrane, displaying both good transparency b and flexibility c. d CIE-
1931 chromaticity diagram showing the emission from the typical g-C3N4/silica gels (marked by the black cross) excited at 365 nm. e FTIR
spectra of AEATMS, g-C3N4-silica gels, and g-C3N4 particles. f Schematic drawing of an AEATMS-capped g-C3N4 particles in the g-C3N4/silica
gels. Reproduced with permission from Ref. [119]. Copyright 2016 Wiley
Nano-Micro Lett. (2017) 9:47 Page 15 of 21 47
123
to practical applications). We believe that more emerging
properties and applications of g-C3N4 are around the cor-
ner. Integrations between experimental research and theo-
retical approaches will advance the research progress of
g-C3N4 to a large extent. As the exploration of g-C3N4 that
is still in its infancy, there are several remaining key
challenges that must be met in near future, including por-
ous nanostructures for the drug loading and delivery,
improvement of electronic conductivity, memory device
fabrication, solid-state lighting, energy conversion, and
wearable sensors.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
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